Blender NodeSocket Error
For example, using the bpy docs, see the base classes (names of the valid socket ID names).

import bpy
import random

max_number = 5
nodes = []
with_fade_out = 2

try: 
    tree = bpy.context.scene.node_tree
except:
    bpy.context.scene.use_nodes = True

group = bpy.data.node_groups.new(type="CompositorNodeTree", name="MultipleSwitch")
group.inputs.new("NodeSocketRGBA", "Default Image")

for i in range(0,max_number):
    group.inputs.new("NodeSocketRGBA", ("Variation - " + str(i+1)))

input_node = group.nodes.new("NodeGroupInput")
input_node.location = (-150, 0)

socket_list = [
    'NodeSocketBool', 'NodeSocketColor', 'NodeSocketFloat',
    'NodeSocketFloatAngle', 'NodeSocketFloatFactor', 'NodeSocketFloatPercentage',
    'NodeSocketFloatTime', 'NodeSocketFloatUnsigned', 'NodeSocketInt',
    'NodeSocketIntFactor', 'NodeSocketIntPercentage', 'NodeSocketIntUnsigned',
    'NodeSocketShader', 'NodeSocketString', 'NodeSocketVector',
    'NodeSocketVectorAcceleration', 'NodeSocketVectorDirection',
    'NodeSocketVectorEuler', 'NodeSocketVectorTranslation', 'NodeSocketVectorVelocity',
    'NodeSocketVectorXYZ', 'NodeSocketVirtual']

for s in socket_list:
    group.outputs.new(s, "Out " + s)

output_node = group.nodes.new("NodeGroupOutput")
output_node.location = (max_number*100 + 100, 0)
group_node = tree.nodes.new("CompositorNodeGroup")
group_node.node_tree = group




you can see here that of all those NodeSocket types, the main ones are:  

NodeSocketBool
NodeSocketColor
NodeSocketFloat
NodeSocketInt
NodeSocketShader
NodeSocketString
NodeSocketVector